This invention relates to electric power supplies, and particularly to a switching power supply capable of a.c. to d.c. voltage conversion, featuring provisions for attainment of closer approximation of the input current waveform to a sinusoidal wave, and a higher power factor, than by the comparable prior art.
A conversion from an alternating to a direct current is possible by a rectifying and smoothing circuit comprising a rectifying circuit having a diode connected to an a.c. power supply, and a smoothing capacitor connected to the rectifying circuit. This type of rectifying and smoothing circuit possesses the disadvantage, however, of a somewhat poor power factor as a result of the fact that the smoothing capacitor is charged only at or adjacent the peaks of the a.c. voltage of sinusoidal waveform. Another drawback is that it is incapable of adjustably varying the d.c. output voltage.
Japanese Unexamined Patent Publication No. 8-154379 represents an improvement of the rectifying and smoothing circuit above. It teaches a switching power supply comprising a rectifying circuit, a smoothing capacitor, a d.c.-to-d.c. converter circuit, and an inductive reactor for a higher power factor. The reactor is electrically connected between the pair of output terminals of the rectifying circuit upon closure of a switch included in the d.c.-to-d.c. converter circuit. The desired improvements in power factor and in input current waveform are thus attained, as the current flowing through the reactor varies in amplitude in step with the a.c. input voltage.
This prior art switching power supply has proved to be not wholly satisfactory, however, particularly in terms of power loss. The advent of a more efficient power supply has been awaited.
The present invention seeks to reduce the power loss, and hence improve the efficiency, of the switching power supply of the kind discussed above, without impairment of its inherent advantages.
Briefly, the invention may be summarized as a switching power supply capable of translating a.c. voltage into d.c. voltage. Included are a rectifier circuit connected to a pair of a.c. input terminals for rectifying a.c. input voltage, a transformer having a primary winding, and a rectifying and smoothing circuit connected between the transformer and a pair of d.c. output terminals for providing d.c. output voltage. The rectifier circuit has at least two output conductors. A smoothing capacitor is connected between one of these output conductors of the rectifier circuit and a first extremity of the primary winding of the transformer. A primary switch is connected between a second extremity of the primary winding and said one output conductor of the rectifier circuit. Also included are a first ancillary winding (e.g. tertiary winding of the transformer) electromagnetically coupled to the primary winding and having a first extremity connected to the first extremity of the primary winding and the smoothing capacitor, a second ancillary winding (e.g. quaternary winding) electromagnetically coupled to the primary winding and to the first ancillary winding and having a first extremity connected to a second extremity of the first ancillary winding, a first inductance coil connected between the other output conductor of the rectifier circuit and a second extremity of the first ancillary winding, and a second inductance coil connected between said other output conductor of the rectifier circuit and the smoothing capacitor and electromagnetically coupled to the first inductance coil. Additionally, and perhaps most characteristically of the invention, there is provided an ancillary switch which is connected in parallel with a serial connection of the transformer primary and the primary switch via the first and the second ancillary winding. A switch control circuit is connected to the primary switch and the ancillary switch for on-off control thereof at a repetition frequency higher than the frequency of the a.c. input voltage. The switch control circuit is adapted to turn the ancillary switch on earlier than the primary switch is turned on, and to turn the ancillary switch off not later than the primary switch is turned off.
Current flows through the first and the second inductance coils as the primary switch is turned on and off at a prescribed switching frequency. The current varies in amplitude in proportion with that of the a.c. input voltage, realizing a higher power factor and a better waveform as in the noted prior art.
Each time the ancillary switch is turned on earlier than is the primary switch, current flows through the first and the second ancillary winding connected in series with the ancillary switch. Since these ancillary windings are in fact the tertiary and quaternary of the transformer, the current flow through these windings results in a discharge of soft-switching capacitance means conventionally connected in parallel with the primary switch, and hence in a drop in the voltage across the primary switch. Thus the primary switch is to be turned on at zero voltage; that is, the so-called xe2x80x9csoft switchingxe2x80x9d is accomplished. The advantages accruing from such soft switching include less switching loss and less noise production.
The primary switch is utilized both for improvements in power factor and waveform and for d.c.-to-d.c. conversion. All these objectives are therefore realized in a device of simple, inexpensive and compact design.
It will also be appreciated that the two ancillary windings for the soft switching of the primary switch can be incorporated with the transformer. The ancillary windings are therefore not to add substantially to the size of the power supply.
According to a further feature of the invention, the ancillary switch has an ancillary diode connected in series therewith for blocking reverse current flow. The diodes constituting the rectifier circuit are thus blocked from high frequency current due to the switching of the primary switch and so prevented from noise production.
A still further feature of the invention resides in a bypass capacitor which is less in capacitance than the smoothing capacitor and which is connected between the two outputs of the rectifier circuit. Such a bypass capacitor will expedite the release of the energy that has been stored on the second ancillary winding, or transformer quaternary.
The two inductance coils are connected in parallel with each other in some preferred embodiments of the invention, and in series with each other in others. Experiment has proved that the power factor is satisfactory either way.
The above and other objects, features and advantages of this invention will become more apparent, and the invention itself will best be understood, from a study of the following description and appended claims, with reference had to the attached drawings showing the preferred embodiments of the invention.